This invention relates to improvements in sheet clamping arrangements for high speed sheet handling drums and, more particularly, it concerns a counterbalancing system by which sheet clamping bars are effectively retained against displacement by centrifugal force during retention of a sheet on a rotatable drum for the performance of high speed printing operations.
In a commonly assigned, copending U.S. patent application Ser. No. 034,665, filed Apr. 6, 1987, an improved sheet clamping system for rotatable drums is disclosed by which the leading edge of each of a succession of sheets to the drum periphery is engaged and clamped in position by a leading edge clamping bar rotatable at all times with the drum, the sheet is drawn past a trailing edge clamping bar capable of retention in a fixed retracted position during drum rotation, the trailing edge of the sheet is engaged by the trailing edge clamping bar for continued rotation with the drum for a printing operation, for example, the sheet is discharged from the drum by the trailing edge clamp and the drum and both clamps are subsequently indexed to receive another sheet. The clamping system disclosed in the copending application features a cam actuating mechanism by which both of the clamping bars are moved between sheet clamping and sheet releasing or retracted positions without interference with use of the drum during rotation thereof for printing or other operations on the drum retained sheet.
The capability of the system disclosed in the aforementioned copending application for sheet loading and unloading operations as well as for accurate indexing of a drum mounted sheet with a printing head, for example, has been demonstrated by testing to a point where the drum system has been established as a candidate for a sheet media carrier for high speed laser reproduction of continuous tone images. In this application, the sheet media, after having been mounted to the rotatable drum, is moved at high speeds in relation to a laser printing head to which digital information is supplied and which is traversed axially of the drum to develop closely pitched spiral recording tracks in the production of a high resolution tonal image on the drum retained sheet. In this type of operation, the drum may be rotated at speeds up to 1600 revolutions per minute during the printing operation and then slowed to very low stepping motor speeds for sheet loading and unloading operations. Also in this type of printing operation, because resolution of the tonal image produced is dependent in substantial measure on proximity of the laser printing head to the drum carried media, the spacing between the drum periphery and the head must be kept as small as possible.
As disclosed in the aforementioned copending Application, the two clamping bars by which a sheet is retained to the drum are supported at their ends by brackets movable in a generally radial direction with respect to the drum axis between retracted and sheet clamping positions. The clamping bars, as thus disclosed, are unsupported except at their ends. As a result of the unsupported length of the clamping bars, at rotational speeds substantially less than 1600 revolutions per minute, centrifugal force acting on the clamping bars tends to cause them to bow outwardly away from the drum periphery. While the ends of the bars may be effective to retain the mounted sheet to the drum periphery, outward movement of the bars creates a problem of interference with a printing head in close proximity to the drum periphery. In addition, the cam actuating system of the sheet handling drum disclosed in the aforementioned copending application is dependent on a clamping action developed by spring force.
Because the clamping bars represent a mass eccentric with respect to the drum axis, two major problems are presented during high speed operation. Firstly, the eccentric mass of the clamping bars operates against spring developed clamped retention force so that the clamping action of the bars against the leading and trailing edges of a mounted sheet is reduced proportionally with rotational speed. Secondly, the eccentric mass of the clamping bars, though relatively acceptable at low speed operation, develop intolerable vibrations at speeds of the class encountered in laser reproduction of tonal images.
There is, therefore, a need for improvement in the clamping system of the type referred to for adaptation of that system to operation at rotational velocities approaching or exceeding 1600 revolutions per minute.